Certain metal-oxide films have been employed to heat the substrate on which they are mounted in applications requiring low-temperature heating, that is, well below 100.degree. F. Most typically, a very thin coating of tin oxide, and particularly stannic oxide, has been deposited by vapor deposition, spraying or the like, on a large area of a glass substrate. The thin film is essentially transparent and yet capable of functioning as a resistance heater if coupled to an appropriate electrical circuit. One application of such glass panels has been to provide frost-free panels for refrigerated display cases of the type frequently used in supermarkets. A very low current can be passed through the tin-oxide film so that a sufficiently elevated temperature of the substrate or inner surface of the panel is created to prevent the condensation of water and the subsequent formation of frost, both of which interfere with the consumer's viewing of products in the display case. Such panels have not been used for heating of the air around the panels in high-temperature applications such as cooking or space heaters.
Glass panels with tin-oxide film deposited thereon also have been used in window glass and oven glass doors. In such applications, the tin-oxide film acts as a passive, infrared, reflective barrier, not as a resistance heater.
U.S. Pat. Nos. 4,970,376 and 5,039,845 also disclose apparatus in which metal-oxide films have been employed as resistance heaters. In U.S. Patent No. 4,970,376, a glass cell used in a spectroscopy device having a relatively small surface area is coated with a thin metal-oxide layer on opposite sides of the cell. The glass cell is a laboratory grade glass, which is heated by resistance heating using the metal-oxide films to a temperature of about 320.degree. F. The resistance heating of the substrate is done in order to enhance the transparency of the cell in the spectroscopy device, not to enable use of the cell as a resistance heating element.
In U.S. Pat. No. 5,039,845, a metal-oxide film is coated on a porous mat of glass fibers. The process employs a vapor deposition which allows the metal oxide film to form on three-dimensional or porous substrates. The primary application of the resulting coated substrate is for use as an electrically conductive plate in lead-acid storage batteries. The patent also describes, however, use of such substrates as resistance heating elements by applying a potential across the coated substrate. An advantage of using the porous fiberglass mats is urged in the patent to be that the resulting heating element would also be flexible. The possible application of such heating elements to culinary purposes, such as warming tables, low-temperature ovens, as well as to de-icing devices and high-temperature heating of gases and liquids is described. Chemical vapor deposition, however, is a relatively expensive process by comparison, for example, to spraying a tin-oxide film onto a substrate.
Further background in connection with the coating of substrates with metal-oxide films and variation of the resistance of such films to the passage of electricity therethrough can be found in U.S. Pat. Nos. 4,349,369 and 4,258,080, respectively.
It is also known that metal-oxide films can be used as resistance heaters in microwave cooking. Thus, various glass and porcelain substrates have had tin-oxide films deposited thereon in various patterns so that when placed in a microwave oven, the film will couple with the microwave energy and produce localized heating of the surface on which it is deposited. In each case, such applications have been limited to containers or food support surfaces that are placed in the microwave oven compartment.
While the patent and other literature have suggested the possibility of using tin-oxide films as resistance heating elements, there are, in fact, no known commercial uses of such devices other than in microwave cooking containers. The various suggestions in the prior art have all had practical drawbacks. Thus, the use of glass substrates tends to require relatively costly, high-temperature, laboratory or PYREX glass. Flexible mats and glass-based sheets have structural drawbacks, and as they are rigidified through various resins and the like, they also can be subject to thermal stress cracking and shattering, particularly at high temperatures. Moreover, expensive chemical vapor deposition techniques may be required for adequate bonding to flexible substrates.
Further, there is a great need for enhanced efficiency of energy conversion in ovens, which typically make very poor use of energy in cooking foods. A Cal rod-type resistance heated oven, for example, typically operates with the rod heating element at about 1500.degree. F. to bring the air temperature in the oven up to useable cooking temperatures, for example, 250.degree. F. to 550.degree. F. Moreover, a 5/16 inch diameter resistance rod-type oven heater will operate at a power density over 40 watts per square inch. The Department of Energy is highly likely to adopt regulations requiring the efficiency of ovens to be noted for consumers on the oven labeling, much as has been done for water heaters, refrigerators and the like. When such requirements are introduced, the extremely low efficiency of ovens using rod-type resistance heating elements will be made readily apparent to consumers.
Accordingly, it is an object of the present invention to provide a resistance heating element suitable for use in an oven having substantially improved efficiency and a greatly reduced power density.
Another object of the present invention is to provide an improved resistance heating element which makes much more efficient use of electrical energy in cooking applications than rod-type resistance heaters.
Still a further object of the present invention is to provide a resistance heating element which is durable, does not pose a safety hazard, has low temperature gradients, and will not be destroyed by thermal stress concentrations.
A further object of the present invention is to provide an oven for cooking of food products which provides a more even heating of the products in the cooking area.
Still another object of the present invention is to provide a resistance heating element which can be used as a highly efficient space heater.
A further object of the present invention is to provide a method of forming a resistance heating element which reduces the amount of energy required to create the heating element.
The heating element, oven and method of the present invention have other objects and features of advantage which will become apparent from, or are set forth in more detail in the following description of the Best Mode of Carrying Out the Invention and the accompanying drawing.